1. Field of the Invention
The present invention relates to a vacuum pump control system, and more particularly to a vacuum pump control system which can control a plurality of vacuum pumps comprising at least one type of cryopump, turbomolecular pump, cryoturbo pump, and dry pump such as a Roots type vacuum pump.
2. Description of the Prior Art
Cryopumps have a first-stage cryopanel cooled to a temperature of 60 K-80 K for condensing water or the like, a second-stage cryopanel cooled to a temperature of 10 K-20 K for condensing a nitrogen gas (N.sub.2), an argon gas (Ar) or the like, and an activated charcoal layer mounted on the second-stage cryopanel for adsorbing at a low temperature a hydrogen gas (H.sub.2) or the like that is not condensed at a temperature of 10 K-20 K. The cryopump is generally used to develop a high vacuum in a vacuum chamber of a semiconductor fabrication facility such as a sputtering apparatus or an ion implantation apparatus.
Semiconductor fabrication facilities usually employ a plurality of cryopumps which are required to be controlled at the same time. A cryopump is a storage type vacuum pump, and hence need to be regenerated, i.e., to release the condensed or adsorbed gases from a cryopanel at periodic intervals. It is therefore necessary to control the cryopumps for regeneration.
Turbomolecular pumps, dry pumps comprising a Roots type vacuum pump, cryoturbo pumps, etc. are also used to develop a high vacuum in a vacuum chamber although they operate by different principles.
Recent semiconductor fabrication facilities have a plurality of vacuum chambers which are associated with respective vacuum pumps that may be of different types depending on processing conditions in the vacuum chambers. It is therefore necessary for the control system of the semiconductor fabrication facility to monitor and control the vacuum pumps of different kinds. The vacuum pumps are controlled by respective controllers which are connected to the control system of the semiconductor fabrication facility.
One system for controlling a plurality of vacuum pumps employs a daisy chain communications network which is a type of computer network. In the daisy chain communications network, a network control unit connected to a host computer is connected to a series-connected array of vacuum pump controllers which control respective vacuum pumps. This system is advantageous in that when the network control unit is apart from the vacuum pump controllers, or the vacuum pump controllers are apart from one another, the number of long cables can be reduced to a minimum.
The daisy chain communications network, however, suffers a problem in that since command signals from the host computer and/or the network control unit cannot be delivered simultaneously to all the vacuum pump controllers, a considerable time is required for, communication between the controllers and control of all the vacuum pumps.
In the event that a communication line is disconnected somewhere between the network control unit and the final vacuum pump controller on the daisy chain or a vacuum pump controller fails or troubles, those vacuum pump controllers which are located downstream of the communication line disconnection or downstream of the failing or broken controller cannot be controlled through the daisy chain communications network. When noise is introduced into the daisy chain communications network at a certain location on the communication line, those vacuum pump controllers which are located downstream of such a location cannot be normally worked.